The '821 patent described an engine that included an air compressor piston and cylinder combination coupled via a crankshaft to a power piston and power cylinder combination. Compressed air from the compressor cylinder flowed through a heat exchanger prior to its introduction into the power cylinder by way of an inlet valve. During the power piston downstroke compressed air flowed into the power cylinder. Fuel was mixed with the compressed air between the inlet valve and the piston in an amount suitable to allow for combustion. During the in-stroke of the power piston, the inlet valve was closed and the exhaust valve was opened to discharge the products of the combustion from the power cylinder through the heat exchanger to release the exhaust heat to the compressed air.